Elizabeth M. Twarog

The WindSat spaceborne polarimetric microwave radiometer: sensor description and early orbit performance

The global ocean surface wind vector is a key parameter for short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. In addition, it affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System Integrated Program Office. It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder, which is planned to provide wind vector data operationally starting in 2010. WindSat is the primary payload on the Department of Defense Coriolis satellite, which was launched on January 6, 2003. It is in an 840-km circular sun-synchronous orbit. The WindSat payload is performing well and is currently undergoing rigorous calibration and validation to verify mission success.

WindSat on-orbit warm load calibration

Postlaunch calibration of the WindSat polarimetric microwave radiometer indicates the presence of thermal gradients across the calibration warm load during some portions of the year. These gradients are caused by reflected solar illumination or eclipse and increase total calibration errors. This paper describes the WindSat warm load and presents the measured on-orbit data which clearly illustrate the anomalous responses seen in the warm load calibration data. Detailed thermal modeling predictions of the WindSat on-orbit performance are presented along with the satellite orbital geometry model with solar inputs in order to explain the physical causes of the thermal gradients. To reduce the resultant calibration errors during periods of anomalous warm load behavior, a correction algorithm was developed which uses the physical temperatures of the gain stages in the receiver electronics to calculate an effective gain. This calibration algorithm is described, and its performance and expected accuracy are examined.

An airborne, real aperture radar study of the Chesapeake Bay outflow plume

An airborne, real aperture radar (RAR) has been used to study the fronts associated with the Chesapeake Bay outflow plume during spring outflow conditions. The RAR produced images of the ocean surface with a range resolution of 10 m, an azimuthal resolution of approximately 30 m, and an image size of 2.5 km × 24 km. Two sampling strategies were utilized: one to synoptically map the entire mouth of the Chesapeake Bay at roughly hourly intervals; and a second to capture the rapid evolution of particular features. In addition, flight times were chosen such that over the course of the entire experiment, data were collected over all phases of the semidiurnal tidal cycle. Three distinct frontal signatures were observed in the imagery. A primary front extended from inside the estuary along the Chesapeake Channel to an anticyclonic turning region east of Cape Henry, and then extended southward along the coast toward Cape Hatteras. This is the classic expression of the plume front, inertial turning region, and coastal jet. A second front with a north-south orientation was observed approximately 20 km east of the bay mouth. This secondary front appears to mark the residual offshore density gradient. A third front was identified east and south of Cape Henry, within 2 km of the coast. This front appears to mark the inshore edge of the plume and has not been documented previously. Time sequences of the imagery indicate that when moving in a clockwise sense around the primary front, the frontal translation speed varies systematically from 20 cm/s in the northern section to 50 cm/s in the south. The position of the primary front and the locations and trajectories of small-scale frontal cusps suggest that bathymetry may be both a significant determinant of the front location as well as a source of along-front variability. These observations are possible due to the airborne RARs ability to collect high-frame rate image sequences, a capability that is not shared by present space-based radar systems.

High resolution polarimetric radar scattering measurements of low grazing angle sea clutter

This paper presents fully polarimetric radar scattering measurements of low grazing angle sea clutter. The measurements were obtained at a three degree grazing angle using a high range resolution (1.5 m) X-Band polarimetric radar operated from a shore site overlooking the Chesapeake Bay. The radar employs pulse-to-pulse switching between orthogonal transmitted polarizations and simultaneously measures two orthogonally polarized components of the backscattered wave to obtain full polarimetric information about the scattering process. The complete Stokes matrix, computed by averaging successive realizations of the polarization scattering matrix, is used to obtain polarization signatures and to determine the polarization dependence of the clutter. Sea spike echoes are shown to be weakly polarized and to exhibit polarization signatures indicative of multiple independent scattering mechanisms. Clutter echoes in the absence of sea spikes are shown to be highly polarized and to exhibit polarization signatures indicative of a single dominant scattering mechanism. >

WindSat Passive Microwave Polarimetric Signatures of the Greenland Ice Sheet

WindSat has systematically collected the first global fully polarimetric passive microwave data over both land and ocean. As the first spaceborne polarimetric microwave radiometer, it was designed to measure ocean surface wind speed and direction by including the third and fourth Stokes parameters, which are mostly related to the asymmetric structures of the ocean surface roughness. Although designed for wind vector retrieval, WindSat data are also collected over land and ice, and this new data has revealed, for the first time, significant land signals in the third and fourth Stokes parameter channels, particularly over Greenland and the Antarctic ice sheets. The third and fourth Stokes parameters show well-defined large azimuth modulations that appear to be correlated with geophysical variations, particularly snow structure, melting, and metamorphism, and have distinct seasonal variation. The polarimetric signatures are relatively weak in the summer and are strongest around spring. This corresponds well with the formation and erosion of the sastrugi in the dry snow zone and snowmelt in the soaked zone. In this paper, we present the full polarimetric signatures obtained from WindSat over Greenland, and use a simple empirical observation model to quantify the azimuthal variations of the signatures in space and time.

The WindSat space borne polarimetric microwave radiometer: sensor description and mission overview

The wind vector affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. Furthermore, accurate wind vector data aids in short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the US Navy and the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office (IPO). It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder (CMIS), which is planned to provide wind vector data operationally starting in 2010

WindSat post-launch calibration

WindSat, an experimental sensor on the Coriolis mission launched in January 2003, is intended to measure the partially polarized emission from the ocean surface, and thereby indirectly measure the ocean surface wind vector. The WindSat calibration and validation process has two primary purposes. The first is to verify the WindSat radiometer absolute accuracy, which is driven by the calibration target accuracy and knowledge, the receiver performance, the antenna characterization and antenna pattern correction, and the geolocation and pointing processing. The second purpose is to validate the sea surface wind speed and direction environmental data products. A key part of this process to identify and quantify error sources and, if necessary, generate new sensor calibration coefficients, algorithms, and environmental data record (EDR) retrievals to bring the data products into specification. This paper focuses on the calibration of the brightness temperatures, the data product used to retrieve environmental parameters such as wind vector

Millimeter wave interferometric radiometry for passive imaging and the detection of low-power manmade signals

Millimeter wave detection and imaging is becoming increasingly important with the proliferation of hostile, mobile millimeter wave threats from both weapons systems and communication links. Improved force protection, surveillance, and targeting will rely increasingly on the interception, detection, geo-sorting, and the identification of sources, such as point-to point communication systems, missile seekers, precision guided munitions, and fire control radar systems. This paper describes the Naval Research Laboratorys (NRL) demonstration broadband passive millimeter wave (mmW) interferometric imaging system. In addition to limited active signal detection, the Ka-band system will provide the potential for detecting the passive signature of non-transmitting hostile systems along with a capability for meter-precision geolocation for imaged objects. The interferometer uses a distributed array of 12 antenna elements to synthesize a large aperture. Each antenna is packaged into an individual receiver, from which a baseband signal is recorded. The correlator is software-based, utilizing signal processing techniques for visibilities, and image formation via beamforming methods.

The WindSat polarimetric radiometer and ocean wind measurements

The wind vector affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. Furthermore, accurate wind vector data aids in short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. WindSat is a satellite-based multi-frequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office (IPO). It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder (CMIS), which is planned to provide wind vector data operationally starting in 2010

WindSat Polarimetric View of Greenland

Abstract : WindSat is the first spaceborne microwave polarimetric radiometer that measures all four elements of the Stokes vector, the brightness temperatures at vertical and horizontal polarizations (TV and TH), and the real and imaginary parts of the cross-correlation of the vertical and horizontal polarization known as the third and fourth Stokes parameters (TU and TF). WindSat was developed by NRL and had been in operation since January 2003. The polarimetric signatures of the third and fourth Stokes measurements are mostly related to the asymmetric structures of the ocean-wind-driven surface roughness. Prior to the launch of WindSat, it was a common belief that land polarimetric signatures at satellite footprint scales would be below the instrument noise level and would not carry any useful geophysical information. However, postlaunch data processing reveals significant land signals in the TU and TF, particularly over Greenland and the Antarctic ice sheets, which are the most environmentally sensitive Earth media, playing a significant role in global sea level and climate changes. Understanding this polarimetric signature, uniquely afforded by WindSat, and its relation with the snow properties and microstructures will have a profound impact on climate study.